RISC-V: Provide stub of setup_profiling_timer()

Fixes the following on allmodconfig build:

profile.c:(.text+0x3e4): undefined reference to `setup_profiling_timer'
Signed-off-by: NOlof Johansson <olof@lixom.net>
Signed-off-by: NPalmer Dabbelt <palmer@sifive.com>

arch/riscv/kernel/smp.c

@@ -38,6 +38,13 @@ enum ipi_message_type {
 	IPI_MAX
 };
 
+
+/* Unsupported */
+int setup_profiling_timer(unsigned int multiplier)
+{
+	return -EINVAL;
+}
+
 irqreturn_t handle_ipi(void)
 {
 	unsigned long *pending_ipis = &ipi_data[smp_processor_id()].bits;